Focused Ion Beam (FIB) microscope systems have been produced commercially since the mid 1980's, and are now an integral part of rapidly bringing semiconductor devices to market. FIB systems produce a narrow, focused beam of charged particles, and scan this beam across a specimen in a raster fashion, similar to a cathode ray tube. Unlike the scanning electron microscope, whose charged particles are negatively charged electrons, FIB systems use charged atoms, hereinafter referred to as ions, to produce their beams. These ions are, in general, positively charged.
These ion beams, when directed onto a semiconductor sample, will eject charged particles, which include secondary electrons, secondary ions (i+ or i−), and neutral molecules and atoms from the exposed surface of the sample. By moving the beam across the sample and controlling various beam parameters such as beam current, spot size, pixel spacing, and dwell time, the FIB can be operated as an “atomic scale milling machine,” for selectively removing, or sputtering, materials wherever the beam is placed. The dose, or amount of ions striking the sample surface, is generally a function of the beam current, duration of scan, and the area scanned. The ejected particles can be sensed by detectors, and then by correlating this sensed data with the known beam position as the incident beam interacts with the sample, an image can be produced and displayed for the operator.
FIG. 1 is a schematic of a typical FIB system. FIB system 10 includes an evacuated envelope 11 having an upper neck portion 12 within which are located a liquid metal ion source 14 and a focusing column 16 including extractor electrodes and an electrostatic optical system. Ion beam 18 passes from source 14 through column 16 and between electrostatic deflection means schematically indicated at 20 toward sample 22, which comprises, for example, a semiconductor device positioned on movable X-Y stage 24 within lower chamber 26. An ion pump 28 is employed for evacuating neck portion 12. The chamber 26 is evacuated with turbomolecular and mechanical pumping system 30 under the control of vacuum controller 32. The vacuum system provides within chamber 26 a vacuum of between approximately 1×10E−7 Torr and 5×10E−4 Torr. If an etch assisting gas, an etch retarding gas, a deposition precursor gas, or some other reactive or non reactive gas is used, the chamber background pressure may rise, typically to about 1×10E−5 Torr.
High voltage power supply 34 is connected to liquid metal ion source 14 and to appropriate electrodes in focusing column 16 and directing the ion beam. Deflection controller and amplifier 36, operated in accordance with a prescribed pattern provided by pattern generator 38, is coupled to deflection plates 20. A charged particle multiplier detector 40 detects secondary ion or electron emission for imaging, is connected to video circuit and amplifier 42, the latter supplying drive for video monitor 44 and also receiving deflection signals from controller 36. A door 48 is provided for inserting sample 22 onto stage 24, which may be heated or cooled. Focused ion beam systems are commercially available from various companies, but the system shown in FIG. 1 represents one possible FIB system configuration.
FIG. 2 shows a sample surface 60 and a raster area 62 where the ion beam is to be rastered. The raster area 62 is rastered by the ion beam being successively aligned with, for example, the 16 sub-areas A to P for pre-determined dwell times. The raster area 62 is usually an endpointing area to be monitored during the ion beam milling operation. During any beam raster operation executed by FIB system 10, which includes imaging, milling, gas assisted etching or deposition, the FIB beam deflection software and hardware deflects the beam in a preset pattern across the surface. At each preset location (sub-areas A to P for example), the beam is left to dwell for a given period of time before moving to the next point in the raster. At its simplest, a raster pass consists of deflecting the beam at fixed increments along one axis from a start point to an end point, dwelling for a fixed dwell time at each point. At the end of a line, the beam waits a fixed retrace time before moving an increment in a second axis. The beam may return to the start point in the first axis and begin again, or may begin “counting down” the first axis from the point it had just reached (depending on whether the raster type is TV-like (the former) or serpentine (the latter). This process continues until all increments in both axes have occurred, and the beam has dwelled at all points in the scan.
In a FIB system, the rastering of an area on the sample can be much more complex than the standard rastering schemes used by imaging devices. In the examples presented here, a standard TV-like rastering scheme is commonly illustrated, but in general, and in particular for gas-assisted etching, the rastering pattern will not be limited to sequential lines. It can, for example produce alternating frames where the lines are interlaced between successive frames to reduce the depletion of the gas used in the process. Another approach might be to mill alternating frames from top to bottom then bottom to top, thereby completely eliminating the need for blanking the beam if serpentine lines are rastered. It is also possible to mill each frame in a spiral or an interlaced spiral pattern. When combined with specific dwell times which may vary within a frame or within a single dwell location, such specialized rastering patterns will be used for specific reasons related to the rate of removal or deposition of material, ie to produce a uniform depth or height; to minimize or maximize the rate of the density of charge deposited on the sample to avoid charge related problems; to maximize the efficiency of the use of gases in gas-assisted processes; to reduce the amount of dead-time when the beam is blanked and is not actively milling the sample, such as during horizontal and vertical retraces in standard TV-like rasters.
It is well understood by those of skill in the art that FIB systems are used to perform microsurgery operations for executing design verification or to troubleshoot failed designs on integrated circuits. This can involve physically “cutting” metal lines or selectively depositing metallic lines for shorting conductors together. Hence, FIB system technologies can enable prototyping and design verification in a matter of days or hours rather than weeks or months as re-fabrication would require. This FIB “rapid prototyping” is frequently referred to as “FIB device modification”, “circuit editing” or “microsurgery.” Such FIB operations will be referred to simply as circuit modification. Those skilled in the art should understand that similar operations to add or remove material on specimens other than integrated circuits may also be desireable, and once again for simplicity will be referred to simply as circuit modification. Due to its speed and usefulness, FIB circuit modification has become crucial to achieving the rapid time-to-market targets required in the competitive semiconductor industry.
During an FIB milling operation, charged particles will be ejected from the material. Those skilled in the art should understand that charged particles emanating from a sample bombarded by an ion beam are detected by the detector and converted into a signal (voltage or current) proportional to the number of detected particles. This signal can correspond to a pixel intensity value for the display. Since the gain of the detector, i.e. the proportionality between the number of incident particles and the output signal, can be varied, it is possible for these systems to detect a wide range of signals corresponding to low and high beam currents, which makes them quite suitable for imaging purposes under a multitude of conditions. Typical beam currents that are used range between 100 pA and 1 nA, with dwell times of about 1 micro-second for example. However, as device technologies shrink, the use of lower beam currents in the range of 1 pA and shorter dwell times make these detection systems less effective.
The use of very low incident beam currents is becoming more widely used in back side circuit edit applications in order to slow down milling processes and avoid exposing or damaging active areas on the silicon itself. High aspect ratio holes or trenches are milled with more accuracy by using low beam currents, as the spot size of the beam can be reduced with a reduction of the beam current. By slowing down the process with low beam currents, it is possible to avoid depletion when using gas assisted etching, to use smaller pixel spacings that can provide better spatially resolved images and, most importantly, to give the operator or expert control system the time to make decisions regarding the process. Furthermore, a reduction of the beam current also minimizes exposure of the sample to the beam while imaging between mills.
An inherent result of using low beam currents is the low yield generation of charged particles, such as ions, which are detected and used for imaging the area being rastered by the beam. Low beam currents may eject several charged particles, however re-absorption into the sample will reduce the number of charged particles that can be detected.
Typical ICs include alternating layers of conducting and dielectric materials with many layers containing patterned areas of both. Consequently, the milling rate and effects of ion beam milling can vary vastly across the device. In most circuit edit operations and other FIB operations, it is preferable to stop the milling process as soon as a particular layer is exposed, this is referred to as endpointing. Imprecise endpointing increases the risk of inadvertently either shorting or opening particular circuits of the IC. Consequently, precise endpoint detection during circuit edit operations is desired. Proper endpoint detection is assessed by the FIB operator, who relies on the visual display of the rastered image of the sample surface and/or graphical data relating to a quantitative analysis of the secondary electron yield over time or a dimension of the sample (such as depth for example). Typically, both the displayed image and the quantitative data results from the same signal source, that being the detected particles ejected from the material when the ion beam impinges on the material surface.
This is a significant problem for current FIB systems, since the electronic circuitry used involves analog to digital conversion of the electrical signal generated by the detection of secondary charges. This is a lossy process, particularly prone to noise at low secondary charge currents. The general circuitry configuration includes a biased detector whose output is connected to an amplifier circuit, which is periodically sampled by an analog to digital convertor (ADC). The sampling period will usually correspond to the FIB dwell time for a particular sub-area being rastered.
A problem with the analog to digital conversion scheme is the reliance on a fixed-bandwidth amplification circuit that is used to accumulate the charge corresponding to the number of detected charged particles and amplify it to be used by further electronics downstream. The particle detector will generate a pulse having a characteristic width in response to a detected charged particle. Therefore, within a dwell time of a sub-area, any number of pulses can be generated for the amplifier circuitry. The amplifier can have a bandwidth that is either high or low, each resulting in deficiencies as will be shown in FIGS. 3a and 3b. If the bandwidth of the amplifier is high, the amplifier output signal will change quickly with each incoming pulse. Then the measured value from the discrete sampling of the analog to digital converter (ADC) will be largely dependent on when the sampling is performed compared to when the incoming pulse occurred as shown in FIG. 3a. When a low bandwidth scheme is used, a single incoming pulse results in a longer duration amplified output, but this results in the visual smearing of the image if the ADC sampling is shorter than the amplified pulse duration: the signal due to a pulse during one dwell time extends into the next dwell time, as illustrated in FIG. 3b. In a system such as an FIB system where the dwell time may vary from 50 ns to several microseconds, it is impractical, if not possible, to properly match the amplifier bandwidth to the measurement system.
FIG. 3a is a graphical illustration of the disadvantages of using an amplification circuit with high bandwidth amplification. High bandwidth amplification is relative to the dwell time of the ion beam, and can categorized as such when the bandwidth (BW) is significantly greater than an inverse of the dwell time of the ion beam (BW>>1/dwell time). FIG. 3a is a graphical plot of voltage (V) versus time, with vertical lines 70 illustrating where in time a charged particle is detected, once corrected for time of flight in the system. In order to simply FIG. 3a, only one dashed vertical line is marked with reference number 70. In the present example, the pulses for sub-areas A and B from FIG. 2 are shown. For sub-area A that corresponds to a dwell time from t0 to t1, five charged particles are detected when a low ion beam current is employed. Each of the detected five charged particles are shown by vertical lines 70, the times at which the charged particles' pulses are detected being spread out in time between t0 and t1. The accumulated charge in the amplification circuit is approximated by voltage curve 72, and the ADC samples at time t1. As shown in FIG. 3a, the accumulated charge in the amplification circuit decays before it is sampled, which is an inherent characteristic of the circuit. For sub-area B that corresponds to a dwell time from t1 to t2, five charged particles are also detected. However, these five detected charged particles are grouped in time near the sampling time of t2. Hence the accumulated charge in the amplfication circuit approximated by voltage curve 74 rises as sampling time t2 is approached. Therefore, the ADC will provide a higher value result in comparison to the sampling at time t1, even though five charged particles were detected from the rastering of both sub-areas A and B.
The resulting analog to digital converted value in each sub-area will correspond to a pixel intensity value for the display image of the raster area. Therefore the pixel(s) of a display image corresponding to sub-area A will differ from that of sub-area B while the same number of charged particles is detected. This will adversely affect the image being generated for the operation, thereby reducing accurate endpointing capability.
FIG. 3b is a graphical illustration of the disadvantages of using an amplification circuit with low bandwidth amplification. Low bandwidth amplification is relative to the dwell time of the ion beam, and can be categorized as such when the bandwidth is significantly less than an inverse of the dwell time of the ion beam (BW<<1/dwell time). FIG. 3b is a graphical plot of voltage (V) versus time, with vertical lines 80 illustrating where in time a charged particle is detected for one sub-area of a raster area. In the present example, a single incoming pulse is detected between t0 and t1, for sub-area A. The accumulated charge in the amplification circuit is approximated by voltage curve for 82. If no charged particles are detected between t1 to t2 and t2 to t3, the amplifier output will peak between t1 and t2 in response to the detected charged particle between t0 and t1, and then decay between t2 and t3. Because ADC sampling will occur at t1, t2 and t3 for sub-areas A, B and C respectively, erroneous data is provided since no charged particles were detected between t1 and t2 and t2 and t3.
Those skilled in the art will recognize that there are methods to alleviate this problem, such as oversampling the amplifier output with a clock that has a higher frequency than the dwell clock and displaying the average value as the pixel intensity. For long duration dwell times, it is also possible to electrically integrate the amplifier output over the duration of the dwell time, but this method is technically challenging for the short dwell times commonly used for gas assisted etching. The technique of oversampling has been employed in the Micrion 2500 focused ion beam microscope.
Although methods do exists to improve the sampling and processing of the amplified analog output, there are other sources of noise inherent to the detector that warrant the use of a pulse counting scheme. In particular, it is well known that the amplitude distribution of the detector pulses corresponding to single event detection (characterized as the signal event response or SER) is quite significant. Although this depends on the technology of the detector, it is quite common that the amplitude of the pulse output by a single secondary particle detection be twice as high as for another detection event. This results in additional image noise (spurious intensity variation between pixels) in analog systems that cannot be removed without degrading other valuable image properties.
The detectors that are used for secondary particle detection are also known to have “dark noise” in the form of low amplitude output pulses that are not due to the detection of a secondary particle but rather due to thermal emission of electrons inside the detector, emission of photoelectrons due to cosmic radiation, etc. These pulses would all contribute to a background level in an analog system. In very low incident beam current conditions, since the number of secondary particles produced is low, reducing the background noise is important for improving the image quality. Those skilled in the art will recognize that the problems described previously are well known and characterized for low intensity signals.
Thus, it is desirable to provide a method and system for improving charged particle detection accuracy at low beam currents.